Fix exception in shutdown
    
    The update_checker-removal patch was borked:
    
    Traceback (most recent call last):
      File "/usr/lib64/calibre/calibre/gui2/ui.py", line 904, in closeEvent
        self.shutdown(write_settings=False)
      File "/usr/lib64/calibre/calibre/gui2/ui.py", line 847, in shutdown
        if self.update_checker:
    AttributeError: 'Main' object has no attribute 'update_checker'